docs(readme): add Codecov badge to README

Author: barissayli

README.md

@@ -1,7 +1,8 @@
 # spring-boot-openapi-generics-clients
 
 [![Build](https://github.com/bsayli/spring-boot-openapi-generics-clients/actions/workflows/build.yml/badge.svg)](https://github.com/bsayli/spring-boot-openapi-generics-clients/actions/workflows/build.yml)
-[![Release](https://img.shields.io/github/v/release/bsayli/spring-boot-openapi-generics-clients?logo=github\&label=release)](https://github.com/bsayli/spring-boot-openapi-generics-clients/releases/latest)
+[![Release](https://img.shields.io/github/v/release/bsayli/spring-boot-openapi-generics-clients?logo=github&label=release)](https://github.com/bsayli/spring-boot-openapi-generics-clients/releases/latest)
+[![codecov](https://codecov.io/gh/bsayli/spring-boot-openapi-generics-clients/branch/main/graph/badge.svg)](https://codecov.io/gh/bsayli/spring-boot-openapi-generics-clients)
 [![Java](https://img.shields.io/badge/Java-21-red?logo=openjdk)](https://openjdk.org/projects/jdk/21/)
 [![Spring Boot](https://img.shields.io/badge/Spring%20Boot-3.4.10-green?logo=springboot)](https://spring.io/projects/spring-boot)
 [![OpenAPI Generator](https://img.shields.io/badge/OpenAPI%20Generator-7.x-blue?logo=openapiinitiative)](https://openapi-generator.tech/)